Printing unit for a printing machine

Abstract

A printing unit ( 1 ) for a printing machine, having an image cylinder ( 2 ), an image generating device ( 3 ) for setting an image on the peripheral surface ( 4 ) of the image cylinder ( 2 ), and an image transfer cylinder ( 5 ) which transfers the image from the image cylinder ( 2 ) to a printing substrate ( 6 ), the image transfer cylinder ( 5 ) having a resilient cover ( 7 ) which exhibits a deformation ( 10, 10 ′) in the force transmission areas ( 8, 9 ), and a transport belt ( 11 ) that carries the printing substrates ( 6 ) and is supported by a back-pressure cylinder ( 12 ) driving the image transfer cylinder ( 5 ) and the latter driving the image cylinder ( 2 ), by friction. The printing unit has a continuous image cylinder speed (v (ü) ). This is achieved by the back-pressure cylinder ( 12 ) being mounted such that it can be displaced with respect to the image transfer cylinder ( 5 ) with a travel-dependent pressing force (F (s) ) which is dimensioned such that the speed of the image cylinder ( 2 ) remains constant with respect to changes in the effective radius (r) for driving the image transfer cylinder ( 5 ).

I claim:  
     
       1. A printing unit ( 1 ) for a printing machine, having an image cylinder ( 2 ), an image generating device ( 3 ) for setting an image on the peripheral surface ( 4 ) of the image cylinder ( 2 ), and an image transfer cylinder ( 5 ) which transfers the image from the image cylinder ( 2 ) to a printing substrate ( 6 ), the image transfer cylinder ( 5 ) having a resilient cover ( 7 ) which exhibits a deformation ( 10 ,  10 ′) in respective force transmission areas ( 8 ,  9 ), between the image cylinder and the image transfer cylinder, and the image transfer cylinder and a transport belt ( 11 ) that carries the printing substrates ( 6 ), and is supported by a back-pressure cylinder ( 12 ) driving the image transfer cylinder ( 5 ) and the latter driving the image cylinder ( 2 ), by friction, comprising: a mount for said back-pressure cylinder ( 12 ), said mount including a device ( 13 ,  14 ) for selectively displacing said back-press cylinder with respect to the image transfer cylinder ( 5 ) with a pressing force (F (S) ), a sensor ( 15 ) to register the substrate thickness, and a control system ( 16 ), the latter determining the pressing force (F (S) ) with the aid of the substrate thickness ( 17 ) such that the speed of the image cylinder ( 2 ) remains constant with respect to changes in the effective radius (r) for driving the image transfer cylinder ( 5 ). 
     
     
       2. The printing unit as claimed in  claim 1 , wherein the device ( 13 ) includes a piston-cylinder arrangement, and wherein the pressing force (F (s) ) provided by the piston-cylinder arrangement of the device ( 13 ) is such that the product of the transmission ratio that results from the respective effective radius (r) and the transmission ratio that results from the respective deformations of the resilient material ( 7 ) in the force transmission area ( 9 ) between the transport belt ( 10 ),—with or without the printing substrate ( 6 ),—and the image transfer cylinder ( 5 ) remains constant. 
     
     
       3. The printing unit as claimed in  claim 1 , wherein the force element ( 13 ) includes a piston-cylinder arrangement which is operated by a fluid or pneumatic medium. 
     
     
       4. The printing unit as claimed in  claim 1 , wherein the force element ( 13 ) includes springs ( 18 ) which act on the back-pressure cylinder ( 12 ) with the pressing force (F (s) ). 
     
     
       5. The printing unit as claimed in  claim 4 , wherein the springs ( 18 ), are selected so as, at least in one characteristic-curve zone ( 20 ), to exhibit a falling force per unit of travel characteristic ( 19 ). 
     
     
       6. The printing unit as claimed in  claim 5 , wherein the springs ( 18 ) have an adjusting element ( 21 ) with which the characteristic-curve zone ( 20 ) can be set as a working zone. 
     
     
       7. The printing unit as claimed in  claim 4 , wherein the springs ( 18 ) are disk springs ( 22 ). 
     
     
       8. The printing unit as claimed in  claim 1 , wherein the force element ( 13 ) is a lever-mounted weight ( 23  and  24 ). 
     
     
       9. The printing unit as claimed in  claim 1 , wherein the force element ( 13 ) is a combination of at least two force-generating elements ( 14 ,  18 ,  22 ,  23  and  24 ). 
     
     
       10. The printing unit as claimed in  claim 9 , wherein the sum of the force/travel characteristic curves ( 19 ) of the force-generating elements ( 18 ,  23  and  24 ) results in at least one falling characteristic-curve zone ( 20 ).